How to keep a project's canonical history correct.

During the mail thread about "Pull is mostly evil" a user asked how
the first parent could become reversed.

This howto explains how the first parent can get reversed when viewed
by the project and then explains a method to keep the history correct.

Signed-off-by: Stephen P. Smith <ischis2@cox.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>